Workers are seen as smoke rises from the North Hyde electrical substation, which caught fire in March - Kirsty Wigglesworth/AP

A power outage that led to the shutdown of Heathrow has been blamed on a fault detected seven years ago but not fixed.

In a damning new report, the National Energy System Operator (Neso) said a substation fire that triggered the incident was caused by a “catastrophic” maintenance failure at National Grid.

It said moisture entering an electric component “most likely” caused the blaze, which started at the North Hyde substation in March and led to the cancellation of thousands of flights at Britain’s largest airport.

Neso said National Grid had been made aware of the moisture issue in 2018.

It said: “This review has seen evidence that a catastrophic failure on one of the transformer’s high-voltage bushings at National Grid Electricity Transmission’s 275-kilovolt substation caused the transformer to catch fire.

“This was most likely caused by moisture entering the bushing, causing an electrical fault.”

The report from Neso has led to Ed Miliband, the Energy Secretary, ordering regulator Ofgem to launch an urgent investigation into National Grid.

He said: “This report is deeply concerning, because known risks were not addressed by the National Grid Electricity Transmission.

“Ofgem has now opened an official enforcement investigation to consider any possible licence breaches relating to the development and maintenance of its electricity system at North Hyde.”

Akshay Kaul, of Ofgem, added: “The North Hyde substation fire resulted in global disruption, impacted thousands of local customers, and highlighted the importance of investment in our energy infrastructure.

“As a result of the report’s findings, we have opened an investigation into National Grid Electricity Transmission. We have also commissioned an independent audit of their most critical assets.

“We expect energy companies to properly maintain their equipment and networks to prevent events like this happening. Where there is evidence that they have not, we will take action and hold companies fully to account.”

Questions over National Grid

The fire in March knocked out three so-called supergrid, high-voltage transformers, shutting down Heathrow for most of the next 24 hours and stranding tens of thousands of passengers.

As part of its findings, Neso also found that National Grid was aware of fault fire suppression systems at the substation, but took no action to fix them.

The latest revelations are likely to raise questions over the management of National Grid and its chief executive, John Pettigrew, who was paid more than £6m last year. He has already announced plans to step down in November.

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National Grid said it had “taken further action” to strengthen its operations since the fire, such as carrying out further risk assessment and re-testing the resilience of substations.

A spokesman said: “There are important lessons to be learnt about cross-sector resilience and the need for increased coordination, and we look forward to working with government, regulators and industry partners to take these recommendations forward.”

A Heathrow spokesman added: “Heathrow welcomes this report, which sheds further light on the external power supply failure that forced the airport’s closure on March 21.

“A combination of outdated regulation, inadequate safety mechanisms and National Grid’s failure to maintain its infrastructure led to this catastrophic power outage. We expect National Grid to be carefully considering what steps they can take to ensure this isn’t repeated.”

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